The present invention relates to borehole logging methods for detecting and characterizing fractures in earth formations and more particularly to a method wherein borehole pressure is intentionally changed between measurements of sonic velocity in order to detect and characterize the presence of formation fracturing.
In many cases, the presence of fractures, either natural or induced, is an important feature governing the production of fluids from a subsurface reservoir. Knowledge of the extent of fracturing and fracture orientation is necessary to plan and evaluate completion, production and stimulation operations. Techniques currently available for characterizing fractures in the vicinity of a borehole involve either core analysis or logging measurements. Collection of core samples is expensive and may not provide the desired information. For example, important large scale fractures may not be sampled in the recovered core. Fractures present in core samples may have been induced by the coring process or by the stress relief associated with bringing a core to the surface. Well logging techniques to identify fractures include high resolution borehole imaging, for example, the borehole televiewer, low resolution borehole imaging, such as circumferential acoustic measurements and dipmeter, and sonic waveform analysis. The various borehole imaging techniques provide only information regarding the zone within about one inch of the borehole wall and do not always distinguish high permeability fractures from features with low permeability. Sonic waveform analysis involves the detection of attenuation or reflection of sound waves from fractures. This technique is most useful for fractures oriented normal to the borehole axis, which prevents effective detection of vertical fractures from vertical wells. In addition, none of these techniques is sensitive to microfractures, which may have apertures of 0.1 mm or less.